1. Field of the Invention
This invention relates to a method and apparatus for driving a flat display panel having a memory function, such as an AC-type PDP (plasma display panel), etc., to allow gradation, i.e. a gray scale, of its visual brightness for each cell.
2. Description of the Related Arts
Flat display apparatus, allowing a thin depth as well as a large picture display size, have been popularly employed, resulting in a rapid increase in its application area. Accordingly, there has been required further improvements of the picture quality, such as a gradation as high as 256 grades so as to achieve the high-definition television, etc.
There have been proposed some methods for providing a gradation of the display brightness, such as Japanese Patent Publication 51-32051 or Hei2-291597, where a single frame period of a picture to be displayed is divided with time into plural subframes (SF1, SF2, SF3, etc.,) each of which has a specific time length for lighting a cell so that the visual brightness of the cell is weighted. A typical prior art method to provide the gradation of visual brightness is schematically illustrated in FIG. 1, where after cells on a single horizontal line (simply referred to hereinafter as a line) Y.sub.1 are selectively written, i.e. addressed, cells on the next line Y.sub.2 are then written. Structure of each subframe SFn on each scanned line, employed in an opposed-discharge type PDP panel, is shown in FIG. 2, where are drawn voltage waveforms applied across the cells on horizontal lines Y.sub.1, Y.sub.2 . . . Y.sub.n, respectively. Each subframe is provided with a write period CYw (or address period) during which a write pulse Pw, an erase pulse Pf and sustain pulses Ps are sequentially applied to the cells on each Y-electrode, and a sustain period CYm during which only sustain pulses are applied.
The write pulse generates a wall charge in the cells on each line; and the erase pulse Pf erases the wall charge. However, for a cell to be lit a cancel pulse Pc is selectively applied to the cell's X-electrode X.sub.i concurrently to the erase pulse application so as to cancel the erase pulse Pf. Accordingly, the wall charge (see FIG. 10) remains only in the cell applied with the cancel pulse Pc, that is, where the cell is written. Sustain pulses Ps are concurrently applied to all the cells; however, only the cells having the wall charge are lit.
Gradation of visual brightness, i.e. a gray scale, is proportional to the number of sustain pulses that light the cells during a frame. Therefore, different time lengths of sustain periods CYm are allocated to the subframes in a single frame, so that the gradation is determined by an accumulation of sustain pulses in the selectively operated subframes each having different number of sustain pulses.
Problem in the prior art methods is in that the second subframe must wait the completion of the first subframe for all the lines creating an idle period on each line. Therefore, if the number of the lines m=400 and 60 frames per second to achieve 16 grades (n=4), the time length T.sub.SF allowed to a single subframe period becomes as short as about 10 .mu.s as an average.
Because T.sub.SF .times.60.times.400.times.4=1 sec. For executing the write period and the sustain period in such a short period, the driving pulses must be of a very high frequency. For example, in the case where the numbers of sustain pulses are 1, 2, 4 and 8 pairs in the respective subframes to achieve 16 grades, the driving pulses must be as high as 360 kHz as derived from: EQU freq.=(1+2+4+8).times.60.times.400=360.times.10.sup.3 Hz.
The higher frequency drive circuit consumes the higher power, and allows less margin in its operational voltage due to the storage time of the wall charge, particularly in an AC type PDP. Moreover, the high frequency operation, such as 360 kHz, may cause a durability problem of the cell. Therefore, the operation frequency cannot be easily increased, resulting in a difficulty in achieving the gradation.
Furthermore, in the above prior art method, a write period CYw of a line must be executed concurrently to a sustain period CYm of another line. This fact causes another problem in that the brightness control, for example, the gradation control to meet gamma characteristics of human eye, cannot be desirably achieved.